The present invention relates to a method for producing an ethylene polymer. More particularly, the present invention relates to a method for producing an ethylene polymer that performs polymerization of ethylene in the presence of hydrogen using a catalyst comprising a chromium catalyst having carried thereon a trialkylaluminum compound.
The ethylene polymer obtained by the method of the present invention is excellent in both environment stress crack resistance (hereinafter, sometimes abbreviated as ESCR) and impact resistance and is suitable for blow molded articles, in particular large blow molded articles.
Ethylene polymers have been used generally and widely as resin materials for various molded articles and are required of different properties depending on the molding method and purpose.
For example, polymers having relatively low molecular weights and narrow molecular weight distributions are suitable for articles molded by an injection molding method. On the other hand, polymers having relatively high molecular weights and broad molecular weight distributions are suitable for articles molded by blow molding or inflation molding.
It has been hitherto known that ethylene polymers having broad molecular weight distributions suitable for blow molding, in particular large size blow molding, can be obtained by use of a chromium catalyst obtained by calcining a chromium compound carried on an inorganic oxide carrier in a non-reducing atmosphere to activate it such that at least a portion of the carried chromium atoms is converted to hexavalent chromium atoms (so-called Phillips catalyst).
However, in recent years, further improvement in quality is desired for ethylene polymers suitable for large size blow molded articles such as a plastic fuel tank and a large size drum. When an ethylene polymer having a broad molecular weight distribution obtained by using a conventional chromium catalyst is blow molded, the molded article does not have a satisfactory balance between ESCR and impact resistance and the demand of customers who desire molded articles excellent in the both properties cannot be satisfied sufficiently.
As a method for obtaining ethylene polymers by using the Phillips catalyst and an organoaluminum compound in combination, there has been disclosed a method for obtaining a polyethylene having excellent ESCR by supplying trialkylaluminum to the reaction system upon polymerization with the Phillips catalyst and performing the polymerization in the co-presence of hydrogen (Examined Japanese Patent Publication No. Sho 49-34759). However, this publication fails to disclose a trialkylaluminum-carried catalyst and a method for obtaining a polymer that has both high ESCR and high impact resistance in good balance, in which the polymerization activity is high.
In addition, methods for obtaining ethylene polymers using catalysts comprising the Phillips catalyst and a trialkylaluminum are disclosed in Examined Japanese Patent Publication Nos. Sho 36-22144 and Sho 47-23668 (U.S. patent application Ser. No. 766,625 filed in 1968). However, these publications fail to disclose a method for obtaining a polymer that has both improved ESCR and improved impact resistance.
Therefore, an object of the present invention is to overcome the problems mentioned above and provide a method for continuously producing an ethylene polymer that has both high environment stress crack resistance (ESCR) and high impact resistance in good balance, suitable for blow molded articles, in particular large size blow molded articles.
The present inventors have made extensive study in view of the above problems and have found that an ethylene polymer that is excellent in both ESCR and impact resistance in good balance by performing polymerization of ethylene in co-presence of hydrogen using a catalyst obtained by having a trialkylaluminum compound carried on a calcination-activated chromium catalyst in an inert hydrocarbon solvent and then removing the solvent followed by drying.
More particularly, the present invention has solved the problems mentioned above by developing the methods for producing an ethylene polymer according to (1) to (7) below and ethylene polymers according to (8) to (10) below.
(1) A method for producing an ethylene polymer, comprising performing polymerization of ethylene in co-presence of hydrogen using a trialkylaluminum compound-carried chromium catalyst, wherein the chromium catalyst is obtained by calcination-activating a chromium compound that is carried on an inorganic oxide carrier in a non-reducing atmosphere to convert chromium atoms in the chromium compound into hexavalent chromium atoms for at least a portion thereof, and treating with a trialkylaluminum compound in an inert hydrocarbon solvent to carry thereon and removing to dry the solvent so that the chromium atoms are not over-reduced by the trialkylaluminum compound.
(2) A method for producing an ethylene polymer, comprising performing polymerization of ethylene in co-presence of hydrogen using a trialkylaluminum compound-carried chromium catalyst, wherein the chromium catalyst is obtained by calcination-activating a chromium compound that is carried on an inorganic oxide carrier in a non-reducing atmosphere to convert chromium atoms in the chromium compound into hexavalent chromium atoms for at least a portion thereof, and treating with a trialkylaluminum compound in an inert hydrocarbon solvent to carry thereon and removing to dry the solvent so that a time in which the chromium compound contacts the solvent is minimized.
(3) The method for producing an ethylene polymer according to (1) or (2) above, wherein the inert hydrocarbon solvent is removed and dried in a time within 3 folds a time in which the carrying reaction of trialkylaluminum compound in the inert hydrocarbon solvent is performed.
(4) The method for producing an ethylene polymer according to (1) or (2) above, wherein the chromium compound-carried inorganic oxide carrier after the calcination-activation has a specific surface area of 350 m2/g or more.
(5) The method for producing an ethylene polymer according to (1) or (2) above, wherein the trialkylaluminum compound-carried chromium catalyst has a molar ratio of the trialkylaluminum compound to the chromium atom of 0.5 to 10.
(6) The method for producing an ethylene polymer according to (1) or (2) above, wherein the polymerization is performed in a liquid phase under the condition under which a ratio of hydrogen concentration (Hc; mass %) to ethylene concentration (ETc; mass %) in the liquid phase satisfies the following formula:
1.0xc3x9710xe2x88x926xe2x89xa6Hc/ETcxe2x89xa61.0xc3x9710xe2x88x922.
(7) The method for producing an ethylene polymer according to (1) or (2) above, wherein the polymerization is performed in a gas phase under the condition under which a ratio of partial pressure of hydrogen (Hp; MPa) to ethylene partial pressure (ETp; MPa) in the gas phase satisfies the following formula:
1.0xc3x9710xe2x88x924xe2x89xa6Hp/ETpxe2x89xa61.0.
(8) An ethylene polymer obtained by the method for producing an ethylene polymer according to (1) or (2) above and having an HLMFR of 0.1 to 1000 g/10 minutes and a density of 0.900 to 0.980 g/cm3.
(9) The ethylene polymer according to (8) above, wherein the ethylene polymer is used for blow molded articles and has an HLMFR of 1 to 100 g/10 minutes and a density of 0.935 to 0.960 g/cm3.
(10) The ethylene polymer according to (8) above, wherein the ethylene polymer is used for large size blow molded articles and has an HLMFR of 1 to 15 g/10 minutes and a density of 0.940 to 0.955 g/cm3.